The therapeutic benefit of many compounds is limited by low uptake of the compound by the target cells or by intracellular breakdown of the compound after uptake. Generally, for maximum therapeutic benefit, delivery of the compound to the cytoplasmic compartment of the cell, where translation of mRNA and protein synthesis take place and where there is a direct link to the nucleus, is desired. For many small, uncharged compounds, permeation across the cell membrane may allow relatively efficient uptake by the cell. However, for a variety of larger and/or charged compounds, such as proteins, nucleic acids, and highly water soluble charged organic compounds, passive uptake by permeation across the cell membrane is more limited. Several methods for improving uptake of such compounds into cells have been proposed. For example, a drug can be administered in modified or prodrug form for transport into cells and then undergo enzymatic conversion to an active form within the cells.
Alternatively, the cellular processes of phagocytosis or endocytosis may be used, where drug-containing particles are engulfed by the cells. However, this approach is limited to certain cell types, for example, phagocytosis is limited to cells of monocyte lineage and to certain other myeloid cells, such as neutrophils, and endocytosis is limited to mesenchymal cells, such as vascular endothelial cells and fibroblasts. Another limitation of this approach is that in the normal course of intracellular processing, particles are exposed to the acidic endosome/lysosome compartments and a host of degradative enzymes, including proteases, lipases and nucleases, resulting in degradation of the therapeutic compound, unless an escape from such processing is engineered into the system.
Still another approach to enhancing drug uptake by cells involves the use of fusogenic particles designed to fuse with the surface membrane of a target cell, releasing the particle contents into the cytoplasmic compartment of the cell. Inactivated and reconstituted virus particles have been proposed for this purpose, particularly in gene therapy where large nucleic acid strands are introduced into cells. Virus-like particles composed of fusion-promoting viral proteins embedded in artificial lipid bilayer membranes are another example. However, safety concerns and the expense associated with growing, isolating, and deactivating viral components limit these approaches.